This invention relates to a current control circuit and, more particularly, to such a circuit wherein a current mirror circuit is selectively controlled so as to establish a corresponding output current. This invention is particularly related to such a current control circuit for use as a selective muting circuit.
In general, a current control circuit is a particular type of circuit wherein an output current which may be supplied to various circuitry, such as an amplifier, a tuner, a modulator, a demodulator, and the like is selectively controlled in accordance with various desired functions or operating conditions. For some applications, it is preferred that some form of isolation exist so that the circuit which generates the output current does not unnecessarily load the controlling circuit, which loading could influence and deleteriously affect the output current value. From this viewpoint, it is a general object of this invention to provide a current control circuit which supplies a load current to a particular load, but the load does not influence the operation or responsiveness of the controlling circuit which is used to control the current generating portion of the current control circuit.
One advantageous application of such a current control circuit is in a muting circuit. To best appreciate the advantages achieved by this current control circuit, an example of a previously proposed muting circuit will be discussed. In my earlier U.S. Pat. No. 4,049,918, a multiplexed stereo demodulator is disclosed for deriving left (L) and right (R) audio information from a received, multiplexed stereo signal. The demodulator includes a differential amplifier having the usual constant current source for amplifying the received monophonic (L+R) signal. The received signal, which also includes the stereo component (L-R) modulated onto a carrier, is amplified by another differential amplifier, this other amplifier also including the usual constant current source. This second differential amplifier is connected to a multiplier, the latter being supplied with an unmodulated carrier, and this multiplier is further supplied with the amplified output derived from the first differential amplifier. The resultant output of the multiplier is the individual L and R channels of audio information. In the event that a muting operation is to be performed, for example, if the multiplexed demodulator is used in an FM broadcast receiver wherein interstation noise is to be muted while the tuning section is tuned from one broadcast station to another, a muting signal is produced and supplied to the aforementioned constant current sources of the individual differential amplifiers. This muting signal effectively deactivates the constant current sources such that no current flows through the differential amplifiers. As is appreciated, this effectively mutes the output of the multiplexed demodulator. When a muting operation no longer is desired, such as when the tuning section of the FM receiver is tuned correctly to a broadcast channel, the constant current sources promptly are reactivated and current once again flows through the differential amplifiers. However, this abrupt initiation of current flow may result in a loud, sudden noise in the loudspeaker system which is supplied from the demodulator. This so-called pop noise is, at best, unpleasant to a listener and may result in damage to the loudspeaker system.
The foregoing problem can be overcome by a "soft" muting control. That is, if constant current flow is reinitiated in a relatively gradual manner, a pop noise would not be produced. It is one purpose of the present invention to provide a current control circuit which can be used to control a muting operation so that "soft" muting can be achieved without the danger of a pop noise.